This invention is concerned with a method of measuring light levels, particularly the measurement of exterior light levels in the zone of approach to a tunnel in which vehicles are moving, and from which exterior light levels the minimum light level to be programed inside the tunnel can be calculated, by means of a photo-sensitive cell installed in a recessed tube so arranged as to sense light levels contained within a cone defined by the center of the cell, which is located along the axis of the tube and which constitutes the apex of the cone, and by at least the open end of the tube.
It is known that the interior light level in a tunnel, particularly in the mouth of the tunnel, must be adapted to the prevailing exterior light level in the zone of approach to the tunnel and that, for users entering the tunnel, this adaptation must maintain the proper level of visibility.
The idea of measuring this exterior light level--generally referred to as the adaptive light level for drivers--in order to calculate the minimum light level to be programed inside the tunnel has previously been conceived according to two distinct methods, to wit:
The first method, known as "L.sub.20.", is one in which the light level in the zone of approach to a tunnel is treated as a constant corresponding to the arithmetic mean of light levels contained within a 20.degree. cone of revolution, which corresponds very nearly to the driver's field of vision, the latter being limited by the edges of the windshield in the driver's vehicle.
This method is implemented by means of a light meter used in the lighting control system for tunnels. The light meter, the response curve of which is constant between 0.degree. and 20.degree., consists substantially of a photo-sensitive cell provided at the back of a tube of circular transverse cross-section, such that the center of the light meter is located on the axis of the tube. The 20.degree. opening of the above-mentioned cone is, on the one hand, defined by the center of the photo-sensitive cell and the rim of the tube delineating the tube's open end and, on the other hand, obtained through the means of a group of diaphragms, with openings of various diameters, aligned in planes perpendicular to the axis of the tube. The open areas of these diaphragms decrease in size along the front-to-back direction inside the tube.
Even though it has the advantage of simplicity, the L.sub.20. method has the disadvantage of lacking a precise physiological response to the phenomenon of light level adaptation by the human eye.
A second method, known as "L.sub.seq ", is based on the fact that it is essential to determine the light level to which the driver's eye is physiologically adapted in the zone of approach to the tunnel in order to calculate the light required inside the tunnel, and particularly in the mouth of the tunnel.
The factor that influences adaptive light level is the light level under haze conditions (L.sub.seq) equivalent to that produced by the various light levels in a vehicle driver's field of vision.
According to known researchers, such as STILES, HOLLADAY, and FRY, said light level L.sub.seq obeys the following law: Light level L.sub.seq is directly proportional to the illumination falling on the eye from various light sources in the eye's field of vision, and inversely proportional to the square of the angle between the direction of view and the light source.
Thus, in measuring light levels that will determine the light level at the mouth of a tunnel, this method involves taking into consideration the various light levels in a vehicle driver's field of vision. A light meter capable of making aforesaid measurement, in accordance with the law stated above, requires a lens (known as a "FRY lens") which is highly complicated and very costly; this has the disadvantage of restricting the application of L.sub.seq method and the use of such a light meter.